As Lady Rose Aldridge née MacClare, the ravishingly feisty Crawley cousin on “Downton Abbey,” Lily James might have appeared the embodiment of a spoiled-rotten stepsister when auditioning for Disney’s “Cinderella.”

And as Daisy Mason, the series’ scullery maid turned assistant cook and budding revolutionary, Sophie McShera might have seemed the ideal candidate for the orphan girl sentenced to cleaning fireplaces and scraping plates until being rescued by Prince Charming. (But not before rescuing him first.) Then Kenneth Branagh, the film’s director, heard Ms. James speak.

“I loved the quality of her voice,” he said in a phone interview from London. “I loved the warmth, tone and range in it. I found it very expressive.” He especially loved that, across the drawn-out process of auditioning and screen testing, “she kept her good humor and she kept her patience and she kept finding a playful quality that in itself seemed to me like a great harbinger of good fortune for the part.”

That’s when he dangled the Swarovski crystal slipper before Ms. James — and later cast Ms. McShera (who, alas, never got to try the Cinderella role on for size) as Drizella, one of the wicked stepmother’s demon spawn — setting their upstairs-downstairs-at-Downton dynamic on its head.

“It was an easier leap of the imagination for me to play an ugly stepsister than the princess,” Ms. James said, her lilting cadence on full display as she explained her initial reluctance to step into the title role of the film, which opens March 13. “I was actually really looking forward to playing the off-center part, to not having to watch how I look or what I say.

“I see it now as the right fit. And in the scenes where I’m tightening Sophie’s corset and cleaning her plate, I think she loved bossing someone around. There was a real sense of gratification that she finally had the upper hand.”

Ah, sweet revenge.

“This time I had all the gorgeous dresses, and I was the one who was speaking properly,” Ms. McShera said giddily, her Yorkshire accent thick. And for an actress whose regular job requires her to go cosmetic-free, save for a smear of fish mousse or turnip purée, the three hours it took to apply Drizella’s ginger ringlets and scarlet Cupid’s bow were a kind of movie-set Nirvana.

“We wore so much makeup,” she said, “that when I washed my face, I nearly didn’t recognize myself in the mirror.”

Harnessing the phenomenon that is “Downton Abbey,” which on Sunday wrapped its fifth season on PBS’s “Masterpiece,” might seem like savvy, even cynical casting.

“For all of their youth, the great bonus was their experience in ‘Downton’ as performers rather than the fact that they happen to be in a hit show,” Mr. Branagh countered. “These were young people who were coming to me having been in front of the camera a great deal. And I knew that these two girls were on a daily basis exposed to Maggie Smith and Penny Wilton and Elizabeth McGovern and Phyllis Logan, and that they had very strong examples of terrific acting” — experience that helped when performing alongside Cate Blanchett, as Cinderella’s stepmother-tormentor, and Helena Bonham Carter, as the fairy godmother. “They turned out to be very responsive as a result.”

Taking their cue from the Perrault folk tale and the 1950 animated Disney film, Mr. Branagh and the screenwriter Chris Weitz set out to conjure a contemporary heroine empowered by the conviction of her choices, however antiquated they might seem. (A 21st-century teenager who doesn’t rebel at being banished to an unheated attic, let alone forbidden to attend the prom? So not happening.)

As for “have courage and be kind,” the mantra that some have suggested reduces Cinderella to an anti-feminist milquetoast, Mr. Branagh likened it to the nonviolent resistance of Martin Luther King Jr., Nelson Mandela and Mahatma Gandhi.

“I liked its simplicity,” he said. “It’s a challenging request to do something very simple, which is also very difficult.”

He added: “I’m proud that a sophisticated, intelligent and passionate girl emerges out of a classical framework where her empowerment is not at the price of becoming like a man. I think it celebrates her specific femaleness in a way that encourages people to be who they are, not necessarily in competition with the opposite gender or with an attempt to be what other people appear to wish them to be.”

Cinderella’s gentle qualities may not be as obviously superhuman as those of some other Disney princesses — say, Elsa’s cryokinetic powers in “Frozen” or the anti-aging qualities of Rapunzel’s hair in “Tangled” — “but I think her inner strength is something anyone can possess,” Ms. James said. “She’s not armed with swords or weapons to be strong. It’s from within, and any child can do that.

“And the journey she goes through at the end, where she realizes that the greatest risk is to be who you truly are — that’s something we all want to be. If only I could have that for myself.”

Their days of living in near anonymity, as both Ms. James and Ms. McShera once claimed to, are nearly over as Vogue and InStyle interviews, a licensing agreement with M.A.C. Cosmetics, and a luxury shoe collection are unfurled. BoxOffice.com recently estimated that “Cinderella,” to be introduced on screens by a new animated short based on “Frozen,” Disney’s 2013 blockbuster, will gross $68 million in ticket sales on its opening weekend in North America.

Then there are the towering billboards, and the requisite kerfuffle about whether Ms. James’s tiny waist was digitally whittled down even further. (She and Mr. Branagh insisted it was merely the magic of bone structure and corsetry.) “It’s incredible, seeing your face three-stories high,” Ms. McShera said. “That’s when you really get a sense of the enormity of the film.”

Sighing with what might have been delight — or was it trepidation? — Ms. James said: “No one recognizes me — ever — and now this. I see myself soaring above Sunset Boulevard and Times Square, and it’s just the weirdest thing.”